1. Field of the Invention
This invention relates generally to gear teeth, in particular herringbone gear teeth and improvements in methods for their manufacture.
2. Description of the Prior Art
Helical gears, which have teeth cut at an acute angle to the axis, are designed to have numerous teeth meshing at all points of rotation and to distribute pressure evenly along the entire length of each tooth. Thus, they provide smooth operation and reliability, and they are ideal for power transmission applications. Because of the angle of the teeth on a single helix gear (also referred to as a twisted spur gear), an axial thrust is created, but by using two opposing helices at complementary angles, induced axial thrust is eliminated.
The performance of a precision gear is largely determined by tooth accuracy and surface characteristics. In many cases, the gear surfaces are case hardened by a carburization, nitriding or similar process and subsequently finish machined to a glossy polish. Case hardening improves wear and corrosion resistance, enhances surface uniformity and purity, and induces a residual compressive stress, leading to significant improvements in gear performance and life. Furthermore, the physical properties of case-hardened steel are extremely good, capable of transmitting twice the torque as through-hardened gears.
Double helical gearing can generally be categorized into two types—the herringbone gear, where the two helices meet in the center to form continuous gear teeth as shown in FIG. 1, and the conventional double helical gear, with a relief gap between the two helices as shown in FIG. 2.
Herringbone gears (FIG. 1) are used as pump rotors and as power transmission gears in applications which require high torque with lower speed, particularly where the face width of the gearing is limited. Although continuous beam double helical gearing has been in use for over fifty years, the hardness of the parts and the tooth dimension accuracy has been limited due to manufacturing difficulties inherent in machining the teeth at the apex.
Herringbone gears have been limited to manufacture by Sykes gear generators or shapers, in which the two opposing helices are machined simultaneously by reciprocating cutters which alternately cut the left and then the right helix with each machine strike. The cutters cut the tooth profile as they stroke to the of the center of the gear face. A limitation to this process is the scarcity of large-pitch high-accuracy Sykes cutters. Further, because these cutters can not cut metals of hardness greater than 35 Rockwell C, the finishing process is limited to through-hardened gearing. The tooth finishing process is equally inaccurate and statistically unpredictable.
Contrarily, conventional double helical gears, as shown in FIG. 2, do not suffer the limitations of herringbone gears. Because the relief gap allows the cutter centerline to travel beyond the end of each helix, a variety of high accuracy machine tools, including some capable of machining hardened steel much greater than 58 Rockwell C, can be used to cut and finish these gears. Therefore, conventional double helical gears with relief gaps are more commonly used in high performance applications than herringbone gears.
Unfortunately for pumping applications, gears with a relief gap are unsuitable for use as pump rotors, so herringbone gears, with their hardness and case depth limitations, must be used. These gears are typically finish cut then nitrided, providing a 0.020 inch to 0.025 inch case. Since nitriding takes place at elevated temperatures, surface irregularities are induced due to distortion. The shortcomings directly affect pump performance. For example, herringbone gears used in plastic melt pumps are subject to case flaking on tooth surfaces due to shallow case depth, overloading pressures, or tooth error from inaccuracies inherent in machining.
A herringbone gear with improved hardening and tooth quality is desired.